In a statement made outside his Mass. Hall office earlier this afternoon, Summers addressed a group of about 150 people, which included a group of student supporters who chanted, "Stay, Larry, stay," and "Five more years."

"This has not been a simple day in my life," Summers told the crowd, many of whom reached out to shake his hand.

Summers wrote in a letter to the Harvard community that divisions between him and the faculty made it "infeasible" to lead, adding that his tenure had been marked at times by "strains and moments of rancor."

"I have reluctantly concluded that the rifts between me and segments of the Arts and Sciences faculty make it infeasible for me to advance the agenda of renewal that I see as crucial to Harvard's future," he wrote. "I believe, therefore, that it is best for the University to have new leadership."

Summers added that "complacency is among the greatest risks facing Harvard."

"I have sought for the last five years to prod and challenge the University to reach for the most ambitious goals in creative ways," he wrote. "As I leave the presidency, my greatest hope is that the University will build on the important elements of renewal that we have begun over the last several years."

Harvard's highest governing body, the Corporation, praised Summers in a letter to the community for bringing "extraordinary vision and vitality" to Harvard.

"Through his tenure as president, Harvard has both invigorated its academic programs and engaged more keenly with the complex challenges facing society. Harvard's paths forward will long bear the imprint of his vision," they wrote. "While this past year has been a difficult and sometimes wrenching one in the life of the University, we look back on the past five years with appreciation for all that has been accomplished and for the charting of an ambitious forward course. We look ahead with confidence in the capacity of our faculty, students, staff, and alumni to embrace a future full of possibility and opportunity."

When Bok assumed the presidency in 1971, he inherited a campus roiled in controversy and deeply divided by the Vietnam War. His predecessor, Nathan M. Pusey '28, came under fire in 1969 for calling police to respond to a student occupation of University Hall.

In his first years as president, Bok took on the role of campus healer, following on his legacy as dean of the Law School, where he was very popular among the students.

"I will do my best to carry out the Corporation's request," Bok, who is 75, said in a University press release. "There is no institution I care about more deeply, and I will make every effort to work with colleagues to further the University's agenda during this transitional period."

Summers plans to return to teaching after a year-long sabbatical, when the Corporation plans to name him one of Harvard's 19 elite University professors, according to the release.

FACULTY REACTS

Professors today reacted with relief that a vicious showdown with Summers at next week's Faculty meeting had been averted.

"I think we have an opportunity to get back to the business of the Faculty," said James J. McCarthy, chair of the Environmental Science and Public Policy degree committee.

"I'm sad and relieved, I guess," said History Department Chair Andrew Gordon '74 moments after learning the news. Gordon, who together with McCarthy coordinates an informal but influential group of department chairs, speculated that the Corporation had made the decision that Summers would resign.

"I get the sense that this resignation and Dean Kirby's resignation are probably similar in that they are forced resignations," Gordon said. "It's just logical."

While what the semester holds remains unclear, many professors said they expected the search for a permanent replacement of outgoing Dean of the Faculty William C. Kirby to be postponed until a new president is named.

Historian Laurel Thatcher Ulrich, who led a group of professors that was preparing for the dean search process, said that she expected the search would be "on hold."

"I can't imagine it can go forward in this situation with an interim president," she said, adding that the Faculty Council will discuss the matter at its meeting tomorrow.

Summers' announcement, unsurprising but still momentous on a campus rocked by controversy, brought some of the president's harshest critics to utter rare kind words.

"I admire him for the humility and the dignity it took to step down," said J. Lorand Matory '82, professor of anthropology and of African and African American studies. "A lesser person wouldn't have done it."

Professor of Economics Edward L. Glaeser, a steadfast Summers supporter over the past year, said he was upset, but not surprised.

"We all expected this for the last week, so it's hardly a huge shock," Glaeser said. He added he hoped the next permanent president would be "someone who holds fast to President Summers' commitment and is, perhaps, a much better manager of academic talent."

"I think it's entirely possible, in 30 years, that oddly, despite all this mess, this will be seen as a very successful presidency that led us into the right direction in terms of renewed commitment to undergraduate education, in terms of setting the stage for Allston construction, and in terms of making Harvard a major player for the life sciences in the 21st century," Glaeser said.